The method and the circuit of the actively transmitting tag is designed for high-frequency communication with an interrogator, whereat the tag's circuit observes a phase of a tag's antenna signal before the tag starts transmitting a data frame. The method and the circuit of the invention are intended for communication according to communication protocol ISO 14443 B with phase inversions at transitions between half-periods of a subcarrier in a transmitted signal as well as according to communication protocol ISO 14443 A with phase inversions at transitions between half-periods of a subcarrier in a transmitted signal in transmissions with bit rates of 212 kb/s, 424 kb/s or 848 kb/s, or according to other communication protocols (e.g. FELICA), in which the actively transmitting tag transmits long data frames, whereat the actively transmitting tag controls a phase of the transmitted signal by detecting a phase of the transmitted signal with regard to a phase of the current antenna signal.
A signal of an actively transmitting tag constructively interferes with an interrogator's transmitted signal at an interrogator antenna. Therefore said tag's signal is detected in a considerably stronger way than a signal transmitted by a passive tag of equal dimensions, which passive tag load-modulates the interrogator signal.
A tiny antenna of the actively transmitting tag manages to build-up a sufficiently strong signal at the interrogator antenna, which accelerates development of actively transmitting tags.
Said constructive interference at the interrogator antenna is accomplished by the actively transmitting tag, which matches a phase and a frequency of the transmitted signal with a received interrogator signal to the best possible extent.
Known technical solutions (Giesecke & Devrient GmbH, Inside Contactless S.A. and IDS d.o.o. together with Austriamicrosystems AG) attain said matching in various ways and have diverse levels of success in practical applications.
Patent EP 1801741 B1 (Giesecke & Devrient) discloses the technical solution, in which the actively transmitting tag matches its transmitted signal to a received interrogator signal in phase and frequency before it starts transmitting a data frame. The phase and the frequency are not corrected during transmitting the data frame, and as a result no reliable communication can be guaranteed.
According to the known technical solution as disclosed in patent EP 2431925 B1 (Inside Contactless S.A.), the phase and the frequency of the tag's transmitted signal are matched to the received interrogator signal before the tag starts transmitting, in fact before each transmitted burst. A packet of eight waves with a carrier frequency of 13.56 MHz represents said burst. In communication according to protocol ISO 14443 A or ISO 14443 B, the burst extends over one half-period of a subcarrier of the transmitted signal.
An actively transmitting tag as disclosed in patent application PCT/SI2012/000024 (IDS d.o.o. together with Austriamicrosystems AG) observes a phase of an interrogator's signal induced in a tag's antenna in such adequately selected time intervals and located even within a data frame being transmitted, in which time intervals, according to a communication protocol, the smart tag does not transmit packets of high-frequency radio waves, and after each such time interval has passed the actively transmitting tag starts transmitting a high-frequency wave packet by exciting its own antenna with the transmitted signal, a phase of which transmitted signal is always set, at the beginning of transmitting each said high-frequency wave packet, shifted with respect to said phase of the antenna signal by the same phase angle.
Said patent EP 1801741 B1 also suggests a way of communicating that the high-frequency wave packet is transmitted with a certain phase during the half-period of a subcarrier of the transmitted signal and in a next half-period with the inverted phase. An amplitude of the tag's signal at the interrogator antenna thus gets doubled and communication range is increased.
Voltage of a transmitted signal at an antenna of an actively transmitting tag is represented in FIG. 1 by a first and second windows for communication according to protocol ISO 14443 A and ISO 14443 B, and by a third and fourth windows for communication according to protocol ISO 14443 A and ISO 14443 B, in both cases by inverting a phase at transitions between half-periods of a subcarrier of the transmitted signal.
The method as disclosed in patent EP 2431925 B1 is not feasible when communicating by inverting a phase at transitions between half-periods of a subcarrier of the transmitted signal either according to protocol ISO 14443 A or according to protocol ISO 14443 B since the first burst is immediately and without a pause followed by a second burst and so on; a half-period of the subcarrier, in which a phase of the interrogator signal would be observed in order to phase-rematch a tag's generator of the transmitted signal, is here no longer available.
The method as disclosed by patent application PCT/SI2012/000024 can be applied in communicating by inverting a phase at transitions between half-periods of the transmitted signal subcarrier according to protocol ISO 14443 A because the actively transmitting tag does not transmit during a time interval with a length of one half of bit or of four periods of the subcarrier. Said method, in turn, cannot be applied when communicating by said inverting the phase according to protocol ISO 14443 B. In this communication protocol the actively transmitting tag uninterruptedly transmits and there is no time window available to match the phase and the frequency throughout the transmission of the complete data frame.